1. Technical Field
The present invention relates to a novel sealing glass composition, in particular, one which comprises solid PbO--ZnO--B.sub.2 O.sub.3 glass and natural garnet as a filler component. This composition has the advantage that thermal strain does not occur at the interface between the crystalline glass and filler because of the excellent chemical affinity. This prevents cracks from propagating at the interface between the main glass and filler, thereby improving the mechanical strength of the glass.
2. Description of Related Prior Art
Crystalline sealing glass compositions used for sealing panels and funnels of TV Braun tubes are known. U.S. Pat. No. 4,131,478 and Japanese Patent Unexamined Publication No. 64-014128, the disclosures of which are hereby incorporated by reference, disclose PbO--ZnO--B.sub.2 O.sub.3 glass with Al.sub.2 O.sub.3 added as a filler component. The composition has a similar thermal expansion coefficient as the panel and funnel of a TV Braun tube over 25 inches in size. The composition also has a high strength at a usage near 440.degree. C. However, since Al.sub.2 O.sub.3 is used as a filler component, a thermal strain occurs at the interface between the main glass and filler due to a large difference between the thermal expansion coefficient of the main glass and filler (the coefficient for the main glass is 105.times.10.sup.-7 /.degree. C. and for the filler is 65.times.10.sup.-7 /.degree. C.). Al.sub.2 O.sub.3, having a chemical stability, does not closely adhere at the interface with main glass component, so the adhesive strength suitable for sealing glass is limited because of the low resistance for crack propagation at the interface. Another problem is that the Al.sub.2 O.sub.3 component is very expensive due to the required particle size of 1-50 .mu.m needed for suitable properties as a filler.
Korean Pat. Unexamined Publication No. 92-635, the disclosure of which is incorporated herein by reference, discloses a crystalline sealing glass composition with a high mechanical strength. The composition consists of a crystalline PbO--ZnO--B.sub.2 O.sub.3 sealing glass powder and Al.sub.2 O.sub.3 --ZnO.sub.2 filler, flowed by heat-treatment for a short time to give properties suitable for a sealing glass for a TV Braun tube. However, this composition has a problem in that a synthetic process is specially required due to the use of Al.sub.2 O.sub.3 --ZnO.sub.2 as the filler component. Another problem is that thermal strain occurs at the interface between the main glass and filler due to the different thermal expansion coefficients of the main glass and filler.
In order to solve the above problems, the present invention uses garnet (Fe.sub.3 Al.sub.2 Si.sub.3 O.sub.2, thermal expansion coefficient of 85.times.10.sup.-7 /.degree. C.) as a filler component, with a thermal expansion coefficient higher than Al.sub.2 O.sub.3, combined with PbO--ZnO--B.sub.2 O.sub.3 as the main glass to essentially prevent crack propagation at the interface between the main glass and filler because of the excellent chemical affinity. The composition may be used for sealing panels and funnels for large CTV over 25 inches in size.